Ah, the cover-up solution to Fermi's paradox. I've heard Mitchell's story before and, as much as I liked the X-Files, I can't entertain a global conspiracy as anything but fancy, heh. We're just not that organized, as a species :P Besides, we're beaming our media signals to the rest of the universe, and once any aliens get a gander at Celebrity Apprentice, I'd like to think they have the sense to be wary of any direct contact.
As to whether humans are the only life forms, Douglas Adams summed it up best. From the Guide:
"4 POPULATION: None
It is known that there are an infinite number of worlds, simply because there is an infinite amount of space for them to be in. However, not every one of them is inhabited. Therefore, there must be a finite number of inhabited worlds. Any finite number divided by infinity is as near to nothing as makes no odds, so the average population of all the planets in the Universe can be said to be zero. From this it follows that the population of the whole Universe is also zero, and that any people you may meet from time to time are merely the products of a deranged imagination."
The Restaurant at the End of the Universe, chapter 19